A leuco chromogen is a chromogen that generates a dye through reaction with hydrogen peroxide in the presence of a peroxidatively active substance such as peroxidase. This type of chromogen, unlike coupling-type chromogens, generates a dye in itself and known leuco chromogens include, for example, phenothiazine leuco chromogens, triphenylmethane leuco chromogens, and diphenylamine leuco chromogens (see e.g., Patent Documents 1 to 3).
The leuco chromogen is often used, as in coupling-type chromogens, in the quantification of an analyte component such as cholesterol or glycated hemoglobin contained in a sample such as serum. Specifically, clinical laboratory examinations often involve: converting an analyte component in a sample to hydrogen peroxide; reacting the generated hydrogen peroxide with a leuco chromogen in the presence of a peroxidatively active substance such as peroxidase to convert the chromogen to a dye; and quantifying the analyte component in the sample on the basis of the absorbance of the generated dye. Particularly, the leuco chromogen is preferably used as a highly sensitive chromogen in the quantification of an analyte component contained only in a trace amount in a sample (see e.g., Non-patent Document 1).
The leuco chromogen is used as a highly sensitive chromogen, as described above, in the quantification of a trace amount of an analyte component in a sample, whereas the leuco chromogen has poor storage stability and undesirably develops color spontaneously with time, particularly, in a solution. To cope with this undesirable poor stability of the leuco chromogen, methods for stabilizing the leuco chromogen in a solution have been studied and reported so far (see e.g., Patent Documents 4 and 5). Unfortunately, these methods for stabilizing the leuco chromogen are not always satisfactory, for example, because they must be performed under strict conditions.